1. Technical Field of the Invention
This invention relates to compositions for and methods of treating subsurface formations. The invention also relates to methods of treating subsurface formations to stimulate the recovery of petroleum hydrocarbons therefrom. The invention further relates to insoluble solid particles coated with materials which are useful in methods of stimulating the recovery of petroleum hydrocarbons from subsurface formations. The invention still further relates to a method of making a viscous liquid having insoluble solid particles suspended therein.
2. Description of the Prior Art and Problems Solved
Petroleum hydrocarbons occur in the pore spaces of identified subterranean formations. Hydrocarbons are recovered from the pore spaces through a borehole drilled from the surface of the earth which penetrates the identified formation. Petroleum flows from the pore spaces through the formation to the borehole and then to the surface. For a variety of reasons the rate of flow of hydrocarbons through a formation can be unsatisfactorily low.
The art has developed a number of processes for increasing the rate of flow. Such processes are sometimes referred to as formation stimulation. One such stimulation process, referred to as hydraulic fracturing, features the use of an aqueous liquid to form a crack, that is, a fracture, in the formation through which formation fluids can flow toward the borehole. The aqueous liquid is referred to as a fracturing fluid.
In the performance of a hydraulic fracturing service, the pressure of the fracturing fluid is intentionally increased within the borehole at the intersection of the borehole and the identified formation, i.e., the formation face. At some time during the increase of the pressure at the formation face the pressure becomes sufficiently high to overcome the mechanical ability of the formation to resist the applied pressure. At that point a fracture opens in the formation and the fracturing fluid flows into the fracture causing it to increase in width and extend into the formation. So long as sufficient pressure is maintained on the fracturing fluid it will operate to prevent the fracture from closing.
Since the purpose of hydraulic fracturing is to increase the rate of flow of hydrocarbons from the formation within the formed fracture toward the borehole, it is necessary to remove the fracturing fluid from the fracture to enable the desired flow. This is accomplished by reducing the applied pressure within the fracture. However, a reduction in applied pressure will not result in a sufficient rate of flow if the fracture closes when the applied pressure is reduced. The art has solved the closure problem by the step of placing a quantity of insoluble solid particles, referred to in the art as proppants, into the formed fracture. In operation, proppants are transported by the fracturing fluid to and placed in the formed fracture. The proppants form a porous pack in the fracture. Upon reduction of hydraulic pressure, proppants in the fracture resist the resulting closing force of the formation to thereby mechanically hold, i.e., prop, the fracture open.
The fracturing fluid suspends the proppants in the fluid while they are being transported to the formation. The suspension is the result of a combination of proppant density, fracturing fluid velocity and fracturing fluid viscosity. In this regard, it is ideally desired that the viscosity of the fluid be low in order to minimize surface pumping pressure while pressure at the formation face is being increased. In contrast, at the moment when the fracture occurs, i.e., breakdown, it is ideally preferred that the viscosity of the fluid rapidly increase to a maximum value as the fracture widens and lengthens, and that the viscosity remain at the maximum value to enable suspension of the proppant in the fracture while applied pressure declines and the fracture closes on the proppant pack.
With applied pressure reduced, accompanied by closure of the fracture on the proppants, it is highly desired, if not necessary, to reduce the viscosity of the fracturing fluid to facilitate flow of the fluid from the fracture to thereby enable the hydrocarbons in the formation to flow in the fracture to the borehole. Such viscosity reduction can be accomplished by the action of a chemical agent on the fluid. The chemical agent is referred to as a breaker.
The art has developed, and continues to develop, compositions and methods of using the compositions in the performance of hydraulic fracturing operations. To this end, a fracturing fluid employed in a hydraulic fracturing service can be a fluid comprised of an aqueous liquid, such as water, to which there is added a material which operates to increase the viscosity of the water, a proppant and a breaker.
It is known that addition of a hydratable organic polymeric material to an aqueous liquid can produce an aqueous product having a viscosity which is greater than the viscosity of the aqueous liquid. The produced aqueous product is at least a liquid colloidal dispersion of the polymeric material in the aqueous liquid, and it can be an aqueous solution of the polymeric material. A liquid colloidal dispersion is referred to as a sol. The hydratable organic polymeric material is sometimes referred to as viscosifying agent or as a viscosifier.
The formation of a sol upon addition of the viscosifier to the aqueous liquid is accompanied by an increase in the viscosity of the aqueous liquid. It is also known to increase the viscosity of the sol by adding a reactant to the sol. It is believed that the reactant causes the dispersed colloidal particles in the sol to form connections to thereby produce a product referred to as a gel. The reactant is conventionally referred to as a cross-linking agent, or, more simply, as a cross linker. In that branch of the petroleum producing arts involving hydraulic fracturing, a “sol” is conventionally referred to as a “gel,” the viscosifier is conventionally referred to as a “gelling agent,” and the product produced by addition of the cross linker to the “sol” is conventionally referred to as a “cross-linked gel.”
The desired result of a successful formation fracturing service is the flow of fluid from the formation. This desired result can be frustrated. In this regard, water insoluble scale, for example calcium and barium sulfate, can, over a period of time, be deposited in natural and formed fractures in subsurface formations. Such scale can reduce, and eventually prevent, flow of fluid from the formation. Accordingly, steps can be taken to remove such scale and/or to prevent or, at least, to reduce the rate of scale deposition. One such preventative measure is to include in the proppant containing fluid a material which slowly dissolves in water present in the formation where it operates to inhibit scale deposition. Such a material is known in the art as a scale inhibitor.
Heretofore, a fracturing fluid having proppants suspended therein has been made from separate ingredients at the location of the well head. Accordingly, water, gelling agent, proppant, cross linker, breaker and scale inhibitor have been transported to the well head and there combined to form a fracturing fluid immediately prior to introduction of the fluid into the borehole.